1. Field of the Invention
This invention relates to a television synchronous receiver usable for a television receiver and a VTR video tuner.
2. Description of Prior Art
Recently, television receivers and VTR video tuners have widely used the so-called electronic tuner which uses a variable-capacitance diode as the tuning element. The electronic tuner is advantageous in that it is of a non-contact type and is therefore free from poor contact problems and is convenient for multifunction uses, such as remote control, because of its electronic control, but there is variation in the characteristics of variable-capacitance diodes and the tuning requires the use of inductances, whereby the manufacturing of such systems is difficult and minimizes automatization.
Hence, it is proposed to use a synchronous receiver which uses no tuning circuits and which is easy to integrate. There are various kinds of synchronous receivers, in which the synchronous carrier recovery method is the most suitable for phase-synchronizing the synchronous carrier with a fine television signal. Such method is called the Costas loop method, but to applying this method to a television receiver, it is necessary to form a frequency pull-in circuit suitable for use in the television receiver.
FIG. 1 is a block diagram of a principal construction of a conventional synchronous receiver, which couples the synchronous carrier recovery method synchronous receiver with a frequency error detection circuit using a differentiator. A Costas loop system synchronous receiver comprises a first synchronous detector for synchronous-detecting the in-phase component of a modulated carrier input, a second synchronous detector 2 for synchronously detecting the quadrature component, low-pass filters 3 and 4 for low-pass filtering the output of each of the two synchronous detectors 1 and 2, a phase detector 5 which voltage-multiplies the outputs of two low-pass filters 3 and 4, a low-pass filter 6 which low-filters the output of phase detector 5, a voltage-controlled oscillator 7 which is controlled by the output of low-pass filter 6, and a phase shifter 8 which shifts the output of voltage-controlled oscillator 7 by an angle of 90.degree. in phase. The frequency error detection circuit comprises a band-pass filter 9 which band-pass-filters the output obtained by low-pass-filtering the output of the first synchronous detector 1, a differentiator 10 which differentiates the output of the band-pass filter 9, a band-pass filter 11 which band-pass-filters the output obtained by low-pass-filtering the output of the second synchronous detector 2, a multiplier 12 which multiplies the output of band-pass filter 11 by the output of differentiator 10, and a low-pass filter 13 which low-pass-filtering the output of voltage multiplier 12, and is coupled to the Costas loop system synchronous receiver at the input of a voltage adder 14.
The Costas loop system synchronous receiver adds a signal to the phase detector 5 comprising in-phase component and a quadrature component obtained from the first and second synchronous detectors 1 and 2 so as to thereby obtain from the phase detector 5 a voltage proportional to a phase error between the modulating carrier of input to the receiver and the output of voltage-controlled oscillator 7, that is, the synchronous carrier, the voltage being fed-back to the voltage-controlled oscillator 7, thereby controlling the phase error so as to be zero. In the frequency error detection circuit, the signal of in-phase component differentiated by the differentiator 10 is voltage-multiplied by the signal of the quadrature component by use of the voltage multiplier 12 so as to thereby obtain a voltage proportional to a frequency error between the modulating carrier and the synchronous carrier, the voltage being superposed by the voltage adder 14 on a voltage proportional to the phase error, thereby reducing the pull-in time of the synchronous receiver.
The conventional example as shown in FIG. 1 has a problem in that the extra frequency error detection circuit must be added and it is difficult to actually constitute the differentiator contained therein.